1. Field of the Invention
The present invention relates to a method of fabricating a through-holed wiring board by forming a through hole with a laser beam.
2. Description of the Background Art
In general, a through-holed wiring board is employed for packaging a hybrid IC or the like with high density. In relation to formation of a through hole in a ceramic substrate of alumina or the like, the following two methods are known in the art:
According to the first method, a through hole is formed in a green sheet, which in turn is fired. According to the second method, a through hole is formed in a fired substrate with a laser beam. In the first method, the cost is increased due to the requirement of a metal mold, and this method is unsuitable for fine wiring due to inferior positional accuracy of the through hole caused by contraction in firing. The second method utilizing a laser beam has no such problems, and is most widely employed at present.
In the second method, however, the through hole is formed by thermal energy of the laser beam, and hence the ceramic material is partially melted and thereafter cooled and therefore inevitably remains in the through hole in a vitreously solidified state which is specific to the ceramic material. Thus, an alteration layer which is solidified after thermal melting is defined in an inner wall portion of the through hole, with a number of cracks caused by thermal stress. When a metallized thick or thin film is formed on this alteration layer, therefore, this film is easily separated.